Boone Gorges shares that BuddyPress 2.8 will require PHP 5.3.x. He shared recent PHP version breakdown for sites running a version of BuddyPress greater than 2.0:
Unknown – 1.08%
5.2 – 3.23%
5.3 – 12.92%
5.4 – 34.44%
5.5 – 22.38%
5.6 – 23.02%
7.0 – 2.92%